1. Field of the Invention
The present invention relates to an automatic threshold control circuit and a signal amplifying circuit, and more particularly, to an automatic threshold control circuit and a signal amplifying circuit for amplifying signals by compensating for the low-frequency response of a photodetector in an optical receiver circuit.
2. Description of the Related Art
In the prior art, two factors have posed problems in a signal amplifying circuit (digital signal reproduction circuit). Namely, the first problem has been the variation of signal amplitude with time, due to factors such as variations in the conditions of signal transmission channels (wired signal transmission channels using metal wires or optical fiber, and wireless transmission channels using electrical or light waves), and the second problem has been the variation in the low-frequency component, i.e., DC (direct current) level, of the signal caused by slow variations in temperature, supply voltage, and the like.
Note that these variations, if left uncontrolled, may cause the discrimination threshold level to deviate outside the amplitude range of the input signal as time elapses, eventually rendering signal reproduction impossible. One possible measure to counter these variations may be to employ differential transmission using complementary signals. However, differential transmission requires twice the number of interconnecting wires, interface circuits, and the like, and therefore the cost becomes high.
In the prior art, one possible method to address the first problem is to detect the signal amplitude and to make a logic decision on the signal level ("1" or "0") by using a threshold level set at the center of the signal amplitude.
Further, in the prior art, to address the second problem, so-called AC (alternating current) coupling has often been used that removes a DC component of an input signal through a coupling capacitor.
However, the recent trend is toward integrating circuits (on a single chip), and since there is a limit to the size of a capacitor that can be formed within a semiconductor chip, it is extremely difficult to fabricate a coupling capacitor capable of providing a sufficient low cut-off frequency within a semiconductor chip; on the other hand, using a discrete component such as a coupling capacitor would lead to increased cost. Furthermore, in the case of a symbol sequence where there is no guarantee that the average value of the input signal is 1/2, there arises the problem that the AC coupling cannot be used.
To address the above-outlined problems, it is desired to provide a signal amplifying circuit (digital signal reproduction circuit) that uses DC coupling which does not use a coupling capacitor, and that is capable of properly accommodating variations both in the amplitude and the DC level of an input signal.
The prior art automatic threshold control circuits and signal amplifying circuits and the problems associated with the prior art will be described in detail later with reference to the drawings.